untill you can give the fish away, start doing 30% water changes twice a week to keep the ammonia levels low and the fish alive.
next, put a add as was suggested, give them away, or find a fish store that will take them for free.
get rid of the platys, every last one.
next, keep a few of the guppies, preferably the males. if you end up keeping one of the females, buy a dwarf catfish of some kind, South American Bumblebee cats rock if you can find one. they will eat the offspring that the guppies produce. make sure it is a catfish that doesnt grow more than 3" though, and dont take the petstores word for it, check on line to make sure it doesnt grow big. and make sure you dont get a Bumblebee catfish...these are also known as asian bumblebee catfish...they get HUGE!!!
finally, after you get rid of those guppies and plattys, add to your school of tetras, they need to be in a group of 5 or more to be happy. if you only keep a few male guppies and no catfish, you can add 4 or 5 tetras to the 3 you already have. if you keep a male and female guppy and a catfish to eat their offspring, then add only a couple more tetras to what you already have.
keeping live bearers like platys,mollies,and guppies is dangerous in small tanks, because the fry are alive and have a HUGE chance of surviving, unlike most egg laying fish.